Quantum DLT 8000 Product Manual page 149

Table 5–2 Command Descriptor Block — Field Descriptions
Field
Operation Code
Logical Unit Number
Logical Block Address
Transfer Length
Parameter List Length
Allocation Length
Description
The operation code specifies the command being requested. The list
of supported SCSI commands and their operation codes are
contained in Table 5–1.
The Logical Unit Number contains the number of the device being
addressed. It must be set to 0. The Logical Unit Number is ignored if
the Command Descriptor Block is preceded by an IDENTIFY Message.
Commands that require additional parameter data specify the length
of the Logical Block Address that is needed. See the specific
command descriptions for more detailed information.
The drive does not support Relative Addressing: it defaults to a value
of 0 which specifies that the Logical Block Address specifies the first
logical block of a range of logical blocks to be operated on by the
command. Relative Addressing indicates a technique used to
determine the next Logical Block Address to be operated on.
The transfer length field normally specifies the number of blocks to
be transferred between the initiator and the drive. For several
commands, the transfer length indicates the number of bytes (not
blocks) to be sent. For these commands, this field may be identified
by a different name.
The Parameter List Length is used to specify the number of bytes sent
during the DATA OUT phase. This field is typically used for
parameters that are sent to a drive (for example, mode, diagnostic,
and log parameters). A parameter list length of 0 indicates that no
data is to be transferred.
This field specifies the maximum number of bytes that the initiator
has allocated for returning data. The host uses this field to limit the
size of data transfers to its own internal buffer size.
Quantum DLT 8000 Tape System
SCSI Commands
5-5